1. Field of the Invention
The invention relates to a process and apparatus for the regeneration of fluidized catalytic cracking catalyst.
2. Description of Related Art
In the catalytic cracking (FCC) process, catalyst, having a particle size and color resembling table salt and pepper, circulates between a cracking reactor and a catalyst regenerator. In the reactor, hydrocarbon feed contacts a source of hot, regenerated catalyst. The hot catalyst vaporizes and cracks the feed at 425C-600C, usually 460C-560C. The cracking reaction deposits carbonaceous hydrocarbons or coke on the catalyst, thereby deactivating the catalyst. The cracked products are separated from the coked catalyst. The coked catalyst is stripped of volatiles, usually with steam, in a catalyst stripper and the stripped catalyst is then regenerated. The catalyst regenerator burns coke from the catalyst with oxygen containing gas, usually air. Decoking restores catalyst activity and simultaneously heats the catalyst to, e.g., 500C-900C, usually 600C-750C. This heated catalyst is recycled to the cracking reactor to crack more fresh feed. Flue gas formed by burning coke in the regenerator may be treated for removal of particulates and for conversion of carbon monoxide, after which the flue gas is normally discharged into the atmosphere.
Catalytic cracking has undergone progressive development since the 40s. The trend of development of the fluid catalytic cracking (FCC) process has been to all riser cracking and use of zeolite catalysts. A good overview of the importance of the FCC process, and its continuous advancement, is reported in Fluid Catalytic Cracking Report, Amos A. Avidan, Michael Edwards and Hartley Owen, as reported in the Jan. 8, 1990 edition of the Oil & Gas Journal.
Modern catalytic cracking units use active zeolite catalyst to crack the heavy hydrocarbon feed to lighter, more valuable products. Instead of dense bed cracking, with a hydrocarbon residence time of 20-60 seconds, much less contact time is needed. The desired conversion of feed can now be achieved in much less time, and more selectively, in a dilute phase, riser reactor.
There has been considerable evolution in the design of FCC units, which evolution is reported to a limited extent in the Jan. 8, 1990 Oil & Gas Journal article. Many FCC designs have evolved and many modifications proposed to permit the units to process heavier feeds and especially with feeds containing more metals. Metals such as Ni and V are poisons in the FCC process, reducing gasoline yields and increasing production of dry gas such as hydrogen and methane.
Some workers have tried to cope with high metals feeds by removing the metals just upstream of the FCC process in a fluidized bed demetallation process. Such units remove metals, but are expensive with a cost and size similar to that of an FCC unit.
Allowing the metals to deposit on the FCC catalyst and then passivating with various additives is effective, but some of the additives, e.g., antimony, are toxic and can make the spent FCC catalyst toxic waste. Many of these additive are more effective for Ni than V.
Metals removal from contaminated FCC catalyst is an option, but it a somewhat difficult and expensive one because the metals levels which can be tolerated on FCC catalyst are relatively low, i.e., FCC catalyst becomes poor catalyst long before it becomes a rich source of metal ore. The DEMET process is designed to remove small concentrations of metal from FCC catalyst and permit reuse of the demetallized FCC catalyst in the FCC process. Such a process is also difficult because the FCC catalyst used is sensitive to many processing steps, i.e., it is easy to remove all the metal and destroy the catalytic activity of the FCC catalyst, while much more difficult to have efficient metals removal coupled with retention of satisfactory cracking activity.
We believed the demetallation approach could be made even more effective if it were possible to increase the concentration of the metal on the solids fed to a demetallation process and/or simplify the demetallation processing by using a more robust solid support for the metal(s) than FCC catalyst. It is interesting to review some work that has done on using something other than FCC catalyst to collect feed metals.
Some workers have added circulating metal getters, which react rapidly with Ni and V compounds in the feed. This approach is effective but dilutes the cracking catalyst.
More effective use of the additive is possible if added in a form where its effectiveness is increased sufficiently that its concentration may be reduced. U.S. Pat. No. 4,980,049, incorporated by reference, taught adding relatively small amounts, on the order of 1 to 5 wt %, of soft alumina having an average particle size of 10 to 40 microns. Such getter additives attrit and migrate out of the unit.
U.S. Pat. No. 4,980,050, incorporated by reference, taught use of large particles of friable alumina, e.g., 100-250 microns, as a getter. The process operated with reducing conditions in the regenerator to minimize migration of metal from the getter additive to the zeolite cracking catalyst.
U.S. Pat. No. 4,875,994, which is incorporated by reference, taught use an elutriable mixture of conventional FCC catalyst and large, coarse particles of demetallizing additive. The additive saw the feed first, in the base of a riser reactor and preferably was decoked in an additive regenerator, run under reducing conditions to minimize formation of highly oxidized and volatile metal species
U.S. Pat. No. 4,895,636, incorporated by reference, taught use of large, coarse getter additives such as alumina or sponge coke, but kept the additive segregated from the conventional FCC catalyst. A separate additive regenerator was provided.
U.S. Pat. No. 5,057,205, incorporated by reference, taught use of an additive which removed SOx and captured metals. Use of small sized additive was preferred, but heavier additives were taught as suitable so long as they were not permitted to accumulate in the regenerator. An elutriating regenerator was shown in FIG. 2, which could be used to elutriate particles of low density from particles of high density. The additive could be either the low or the high density particle. A separate additive regenerator 11 is shown in FIG. 1.
We reviewed the work that others had done with a view to improving the ability of the FCC unit to crack heavy, metals laden feeds. We wanted to deal with the problem of too much Ni and V in the FCC feed but without diluting the catalyst. We wanted to avoid the cost of a separate additive regenerator. We wanted to be able to recover the metals in the FCC feed in a more concentrated form so that these metals could become a potentially valuable product rather than a disposal problem.
We discovered a way to crack high metals feeds, without unduly contaminating the FCC catalyst, by taking several steps backward in the art. We used a metals getting additive but kept it out of the riser where the high metals feed was added by forcing the additive to accumulate in the regenerator. We ran our regenerator under highly oxidizing conditions, which promote formation of highly oxidized vanadium species which could readily attack zeolite structure like a cancer. Finally, we forced our additive to stay in the regenerator a long time, rather than attrit or be swept out of the regenerator. This unlikely set of conditions led to a process permitting effective demetallation of the FCC catalyst and efficient recovery of much of the FCC feed metal, especially the V content of the fresh hydrocarbon feed, without diluting the FCC catalyst charged to the riser.